Nitrofurantoin and nitrofurazone, drugs used in the treatment of anaerobic bacterial infections, were detected as mutagens in the Ames Salmonella mammalian enzyme mutagenicity test system. Bacterial nitroreductases are important in the mutagenic activation of these drugs since no mutagenic response was obtained from bacteria deficient in nitroreductase activity. The addition of rat liver 9,000 x g supernatant to nitroreductase deficient bacteria under anaerobic conditions however produced mutations. Thus, under anaerobic conditions mammalian liver enzymes convert these two drugs to mutagenic compounds. Guinea pig testicular microsomes, unlike rat liver, could not activate these drugs to mutagens.